1. Field of the Invention
The present invention relates to a secondary surveillance radar system which has operating functions of the mode A/C and the mode S, and performs surveillance of aircraft and air-to-ground data link communications.
2. Description of the Related Art
In the field of the secondary surveillance radar (SSR), the mode S has been developed recently as a next-generation operating mode. The mode S provides a function of performing air-to-ground data communications in addition to a function of performing selective addressing of an aircraft for individual inquiry/response to the aircraft. Particularly, the SSR that has the mode-S function in addition to the conventional mode A/C function dedicated to surveillance is called a mode-S sensor.
A typical way of implementing the mode-S sensor is to, as with the conventional SSR dedicated to the mode A/C, form an antenna beam in a horizontal plane by using an antenna system mechanically rotating at constant speed, and to divide a time that the antenna beam illuminates an aircraft target (called a beam dwell time) into respective periods allotted for general inquiry/response in the mode A/C and the mode S and individual inquiry/response in the mode S.
The mechanical rotation type of mode-S sensor is excellent in surveillance capability because it permits an antenna beam to make circular scanning. However, it takes a relatively long time for an aircraft target to enter the illuminating range of the antenna beam because of constant-speed rotation, and thus data must be stored until communications have been made possible. For this reason, the limitations on data communications capacity and the time difference between the occurrence of a request for communications and the completion of the communications become great problems.
To solve the problems, an electronic scanning system has been devised. With this system, antenna arrays are arranged in the shape of a cylinder to form an electronic scanning antenna. The electronic scanning antenna permits a beam to be directed to a desired aircraft target at any point of time to thereby perform general, individual inquiry/response with the aircraft target. The system is discussed in detail in a research report on electronic scanning version of radar antennas, published by Japan Civil Aviation Promotion Foundation, March 1986.
It will be expected that the electronic scanning type of mode-S sensor offers significant improvements in the data communications capacity and the delay time characteristic. However, horizontal beamwidths are not uniform in angle of elevation because the antenna arrays are arranged in the shape of a cylinder. Thus, depending on angles of elevation, unwanted responses increase, and monopulse angular measurement characteristics that are requirements of the mode S may deteriorate.
Such problems could be eliminated by making the diameter of the cylindrical array large, say, more than ten and several meters, and using a large number of arrays to narrow the array spacing. Such is almost impossible to realize physically and economically.